The present invention relates to a metering apparatus for polyurethane foam arising from the reaction of two reactants fed separately via separate feed orifices into a mixing chamber formed from a bore provided axially in a core of self-lubricating synthetic material, which core is mounted in a hollow metallic cylinder and provided over a first length fraction with an angular narrowing around which is fitted a metallic sleeve on which a tightening member acts, in such a manner as to subject a second fraction of core of self-lubricating synthetic material surrounding the feed orifices to a compressive prestress at right angles to these orifices, so as to improve the seal of the joint formed by the bore and a valve rod sliding inside the bore under the action of an actuator in order to actuate the flow of two reactants flowing through the feed orifices into the mixing chamber, and comprising a solvent receptacle disposed in the extension of the mixing chamber and traversed by the valve rod.
The reliability of a polyurethane foam metering apparatus depends mostly on the perfection of the sealing joint between the valve rod and the feed orifices of the mixing chamber. The slightest infiltration of reactants between the Teflon.RTM. core and the valve rod, when the latter is in the closing-off position, causes the valve rod to jam and the feed orifices to be obstructed because of the tendency of the reactants to react with each other instantaneously.
Thus, there is known from document U.S. Pat. No. 4,568,003, a metering gun for polyurethane foam arising from the reaction of polyol and a polyisocyanate separately feeding a mixing chamber.
The mixing chamber is constructed by means of two half-shells made from stainless steel which are capable of withstanding the mechanical stresses exerted during the operation of the metering gun. The first and second half-shells are disposed in colinear alignment in order to form an internal longitudinal cavity. A core made from Teflon is inserted from one side of the cavity and is retained inside the cavity by means of the walls of the second half-shell and by means of a retaining ring.
The mixing chamber has two feed orifices via which two fluid components are fed separately into the said mixing chamber. A valve rod slides inside the bore in order to actuate the flow of the two reactants flowing through the feed orifices towards the central bore of the core and in order to guide the stream of foam spurting out of the mixing chamber.
It enables, in a first position, the reactants to flow through the said first and second orifices and, in a second position, prevents the flow of the reactants through these orifices. The valve rod is actuated forwards or backwards by means of a pneumatic, electric or hydraulic actuator detachably fixed to the valve rod by means of a laterally fitting coupling part.
The core is maintained under a compressive prestress applied along an axial and radial direction via the walls of the second half-shell and via the retaining ring, this prestress being exerted by means of a series of dish-shaped washers, called Belleville washers, maintained in their appropriate position by means of a lip formed at the contact end of the two half-shells. The core is provided with an axial bore longitudinally traversing this bore and in communication with the outside of the mixing chamber and with the reservoir formed on the other side of the cavity defined by the first half-shell. Inside the reservoir is a cleaning fluid or solvent, such as a cellulosic solvent for the polyurethane foam.
By virtue of the compressive prestress exerted on the Teflon.RTM. core, a perfect seal is long-lastingly ensured between the bore and the valve rod. This seal prevents any infiltration of reactants into the mixing chamber but, however, it does not manage to prevent a gradual saturation of the solvent with reactants and with foam.
From document EP-A-0,068,670, there is known an apparatus for dispensing a mixture of liquid reactants fed separately via separate feed orifices into a mixing chamber formed from a bore provided axially in a cylindrical core of self-lubricating synthetic material. The core is provided over a first length fraction with an annular narrowing around which is fitted a metallic sleeve on which a tightening member acts, in such a manner as to subject a second fraction of core of self-lubricating synthetic material surrounding the feed orifices to a compressive prestress at right angles to these orifices so as to improve the seal of the joint between the bore and a valve rod sliding inside the bore under the action of a actuator in order to actuate the flow of two reactants.
The tightening member is a clamp held in place by a pair of screws. Such a clamp ensures the correct orientation of the core of synthetic material in the extension of the hydraulic actuator only when the two screws are fully tightened. It has the drawback that a further tightening of the clamp with a view to adjusting the compressive prestress of the synthetic core is no longer possible.
From document U.S. Pat. No. 3,945,569, there is also known a metering apparatus for polyurethane foam arising from the reaction of two liquid reactants fed separately via separate feed orifices into a mixing chamber bored out in a core of self-lubricating synthetic material, which core is tightly held in a cylindrical metallic cavity by means of a cover non-detachably fixed to the abovementioned cavity by means of plastic deformation. The core has a collar on which the cover acts and bears on a non-demountable cartridge disposed in its extension and compressed at right angles to the feed orifices.
The mixing chamber forms a tamper-proof unit in which the compressive prestress is imposed once and for all during the plastic deformation of the cover.
Further tightening of the cover with a view to adjusting the compressive prestress is not possible.
There is known from document EP-A-0,204,577, a polyurethane foam metering apparatus comprising a mixing chamber and a metering rod which are easily demountable. The mixing chamber is formed from a bore, axially provided in a core of self-lubricating synthetic material, having a tapered shoulder in which two diametrically opposed feed orifices are provided. The core is housed, by tightening, in a cavity of the metering apparatus of corresponding shape.
A valve rod slides inside the bore of the core in order to actuate the flow of two reactants towards the mixing chamber and in order to guide the stream of foam spurting out of the latter.
The core of self-lubricating synthetic material is subjected to a compressive prestress perfectly aligned along its longitudinal axis whatever the degree of tightening, by means of a cap provided with an internal thread screwed over one end of the abovementioned cavity of the metering apparatus provided with a corresponding external thread. The core is maintained at right angles to the feed orifices under a compressive prestress perpendicular to the shoulder. Furthermore, the core comprises a metallic aligning ring enabling the valve rod to be guided and any seizing to be avoided.
However, the free end of the core made from deformable synthetic material emerges into the free air far from the tightening cap and does not manage to prevent the stream from spurting or suddenly deviating.